This application claims the priority of Korean Patent Application No. 2003-8007, filed on Feb. 8, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a fuel cell, and more particularly, to a composite electrolyte membrane for a fuel cell and a fuel cell containing the same.
2. Description of the Related Art
Fuel cells are electrochemical devices which generate electrical energy through electrochemical reaction of fuel and oxygen. Because they are not subjected to the thermodynamic limitations of the Carnot cycle, their theoretical power efficiencies are very high. Fuel cells may be used as sources of electric power for industrial, domestic, and automobile driving applications, as well as for electric/electronic products, in particular, portable devices.
Currently known fuel cells are classified into a polymer electrolyte membrane (PEM) type, a phosphoric acid type, a molten carbonate type, and a solid oxide type according to the type of electrolyte used in the cells. If the type of electrolyte is changed, the operation temperature and materials of constitutional elements of a fuel cell are changed.
Fuel cells are also classified into an external reforming type and an internal reforming type according to fuel feeding process. External reforming fuel cells convert fuel into a hydrogen-rich gas using a fuel reformer before the fuel is delivered to an anode. Internal reforming fuel cells, also known as direct fuel cells, allow gaseous or liquid fuel to be fed directly into an anode.
A representative example of direct fuel cells is a direct methanol fuel cell (DMFC). In the direct methanol fuel cell, an aqueous methanol solution is mainly used as fuel and a proton-conducting polymer electrolyte membrane is used as an electrolyte. Because the direct methanol fuel cell removes the need for an external reformer and has excellent fuel handling property, it can more easily overcome the problem of miniaturization than other fuel cells.
Electrochemical reactions involved in the DMFC include an anode reaction for oxidizing fuel and a cathode reaction for reducing protons and oxygen. These reactions are summarized as follows:
Anode reaction: CH3OH+H2O→6H++6e−+CO2 
Cathode reaction: 1.5O2+6H++6e−→3H2O
Overall reaction: CH3OH+1.5O2→2H2O+CO2 
As shown in the above reactions, methanol and water react with each other to produce carbon dioxide, six protons, and six electrons at an anode. The generated protons travel through a polymer electrolyte membrane to a cathode. At the cathode, the protons, electrons from an external circuit, and oxygen react to produce water. Through these reactions, a large portion of the energy corresponding to the heat of combustion of methanol is converted to electrical energy.
The proton conducting polymer electrolyte membrane acts as a channel through which the protons generated by an oxidation reaction at the anode can be transferred to the cathode. At the same time, the polymer electrolyte membrane acts as a separator to separate the anode and the cathode. The polymer electrolyte membrane must have high ionic conductivity to rapidly transport a large amount of protons. In addition, the polymer electrolyte membrane is required to have electrochemical stability, mechanical strength as a separator, thermal stability at an operating temperature, easy thin film formation property to diminish the overall electrolyte resistance for ion conducting, and resistance for swelling by liquid fuel.
A polymer electrolyte membrane is generally made of a perfluorosulfonated polymer called Nafion (a trade name of Dupont). The perfluorosulfonated polymer has a fluorinated alkylene backbone and a sulfonate-terminated, fluorinated vinyl ether side chain. Such a polymer electrolyte membrane is hydrophilic and is ionic conductive in the presence of an appropriate amount of water.
When an aqueous methanol solution as fuel is fed into an anode of a direct methanol fuel cell, unreacted methanol diffuses through a polymer electrolyte membrane to cathode, thereby causing methanol crossover. For that reason, the performance of a fuel cell is significantly decreased. Therefore, reduction of the amount of unreacted methanol is required to decrease the methanol crossover. For this, generally, an aqueous methanol solution with a low concentration of 6 to 16% by weight has been used. However, such an aqueous methanol solution fuel of low concentration reduces the operation efficiency of a fuel cell. In addition, methanol can still permeate a polymer electrolyte membrane, and the poisoning of a cathode catalyst occurs, thereby decreasing the operating life of a fuel cell. These problems may also occur when other polar organic compounds besides methanol are used as fuel.
Therefore, many efforts have been made to decrease the crossover of polar organic fuel such as methanol and ethanol.
U.S. Pat. Nos. 5,409,785; 5,795,668; 6,054,230; 6,242,122; 5,981,097; and 6,130,175 disclose multi-layered electrolyte membranes.
U.S. Pat. Nos. 5,795,496; 6,510,047; and 6,194,474 disclose heat resistance polymer electrolyte membranes.
U.S. Pat. Nos. 5,919,583 and 5,849,428 disclose electrolyte membranes containing inorganic particles for the conduction of protons.
U.S. Pat. No. 4,985,315 discloses an electrolyte membrane containing an amorphous material for the conduction of protons. U.S. Pat. No. 5,672,439 discloses a fuel cell comprising dual catalyst layers.